So, unfortunately oomd uses "io.system." rather than "io.systemd." as
prefix for its sockets. This is a mistake, and doesn't match the
Varlink interface naming or anything else in oomd.
hence, let's fix that.
Given that this is an internal protocol between PID1 and oomd let's
simply change this without retaining compat.
Compared to PID1 where systemd-oomd has to be the client to PID1
because PID1 is a more privileged process than systemd-oomd, systemd-oomd
is the more privileged process compared to a user manager so we have
user managers be the client whereas systemd-oomd is now the server.
The same varlink protocol is used between user managers and systemd-oomd
to deliver ManagedOOM property updates. systemd-oomd now sets up a varlink
server that user managers connect to to send ManagedOOM property updates.
We also add extra validation to make sure that non-root senders don't
send updates for cgroups they don't own.
The integration test was extended to repeat the chill/bloat test using
a user manager instead of PID1.
